Electrooptical transmission



2 Sheets-Sheet l L. G BRHAM Filed Feb. 5, 1927 ELBCTROOPT ICAL TRANSIIIS S ION March 19, 1929.

March 19. 1929- L; s. ABRAHAM Y ELECTRQOPTICL TRANSIISSION 2Sheets-Sheet 2 Filed FGb. 5, 1927 QJ. Kulm' m m" m" QN n Tlll PatentedMar. 19, 1929.

UNITED STATES PATENTv QFFICE' y LEQNARD G. ABRAHAM, 0F LEONIA, NEWJERSEY, ASSIGNOR TO AMERICAN TELE- PHONE AND TELEGRAPH COMPANY, ACORPORATION OF NEW' YORK.

LECTROOITICAL TRANSMISSION.

Application led February 5, 1927. Serial No. 166,242.

This invention relates to the electrical transmission of signals, andparticularly to.y

electro-optical signal receiving apparatus.

An object of the invention is to produce at a receiving point faithfulfacsimiles of pictures, objects, images, messages, etc., which aretranslated into electric current at the transmitting point fortransmissionto said receiving point.

Another object of the invention is to provide a system for'obtaining areciprocal relationship between input and output Waves, which system maybe used With the type of light valve hereinafter described.

rl"he invention in its principal aspects is a light valve which may beused in the production of facsimiles of pictures, objects, etc., and asystem in which such a light valve may be used.

This light valve accomplishes the purpose of the common iris diaphragmwithout the large inertia that is present in the latter. It consists,specifically stated, of a flat ribbon suspended in a constant magneticfield betiveen a constant source of light and the recording film. Thelight passes through a small opening in the ribbon and is focused on thefilm. The received picture current is rectified and that portion of therectified current is passed through the ribbon which will cause thelatter to be initially deflected toward the light sourcel and tooscillate in accordance with the current variations while maintaining a`deflection toward the light source. The degree of exposure of the filmis greater when the distance from the ribbon to the light sourceis'decreased and is propor tional to the square of the current passingthrough the light valve ribbon. Thel received current before beingimpressed upon the light valve is transformed into a similar current ofamplitude equal to the square root of the original current. The degreeof exposure of the film is therefore proportional to the pic-turecurrent as received from the transmitter.

it is desirable to have the light source very close to the ribbon. inwhich case it maybe advantageous to initially deflect the ribbon awayfrom the light source rather than tovvard it. This may be accom ilishedby utilizing the proper portion of the rectified received current. Inthis case the degree of exposure of the sensitive film is proportionalto the reciprocal of the picture current as received from thetransmitter. To correct for accompanying drawings, in whichsimilarreference characters designate corresponding parts in the severalviews, and in which:

Figure lis a diagrammatic illustration of a picture transmission systememploying the electro-optical system of the invention;

Fig. 2 is a side view of the light valve employed in the electro-opticalsystem;

AFig. 3 is a diagram for reference in connection with the description ofthe electro-optical system; and

Fig. 4 is a diagrammatic illustration of a picture transmission systemsimilar to that shown in Fig. .1, employing the translating system oftheinvention.

The transmitting terminal shown in Fig. 1 is similar to that disclosedin Patent No. 1,667,805, May 1, 1928 of H. E. Ives. This terminalcomprises a modulator 5 supplied with carrier waves, which may be of1300 cycles frequency, from a source 6 and with picture current from aphotoelectric cell 7 and picture current amplifier 8. Modulated carrlerwaves from themodulator 5 are impressed upon the transmission l1ne ML'through an output transformer 18 and a band the transparency of thepicture through an image of this aperture is focused by the lensaperture in screen 14. The amount of light aecting the photoelectriccell-7 is determined by the tone value or density of the elemental areaof the transparency 11 which is at that instant adjacent the aperture inthe screen 14 at the focal point of the light beam. The amount of Jlightwithin the photoelectric cell 7 in turn determines the amount of currentwhich can flow from the battery 15 through the photoelectrie cell 7 andresistance 16 and consequently the potential drop across the resistance16, which potential is impressed upon the input circuit of the picturecurrent amplifier 8. The amplified picture current is impressed upon themodulator 5 by resistance 17 simultaneously with carrier current fromthe source 6. Modulatedf carrier Waves from the modulator 5 areimpressed upon the transmission line ML for transmission to the distantreceiving terminal.

At the receiving terminal the incoming modulated carrier waves passthrough a band filter BF2 into a translating circuit 19 of' the typedisclosed in a copending application of G, Crisson, Serial No. 737,415,filed September 12, 1924, having the property of producing a current theamplitude of which varies as the square root of the amplitude of thereceived current. The purpose of the circuit 19 will appear hereinafter.It is evident that if a current i is received from the line ML thecurrent in the output of the translating circuit 19 may be designatedFor convenience, in the following description, this latter current will`zbe called L.. The band filters at the transmitting and receivingterminals serve to pass only the picture modulated carrier current andto suppress currents of other frequencies. Among the other currentspresent on the transmission line ML which are suppressed by thesefilters, is a control current which is usually employed in systems ofthis general type to effect synchronization of the transmitting andrecording apparatus. The apparatus for transmitting and receiving suchsynchronizing current forms no part of the present invention and henceis not shown in the drawing.

The output of the translating circuit 19 is p coupled by means of atransformer 2O to the input circuit of a rectifier 21 whereby thecurrent I n is rectified and passed through a flat ribbon 22 of thelight valve which forms a part of the electro-optical systeln. Asuitable const-ant source of light 23 evenly illuminates a considerablearea on the ribbon 22. In the center of the ribbon is a small circularopening 24 which allows the light to pass through to the lens 25 whichdirects it to the screen 26 having a central aperture, the beam of lightat this point always being somewhat greater than the aperture in thescreen. The

`2.7 on the light sensitive fihn 28 which is wrapped around the drum 29.

The light valve includes a winding 30 mounted on the central core of amagnetic frame 31. A magnetic plate 32 connected to the frame 31, asshown in Fig. 2, provides an air gap in which the ribbon 22 issuspended. Circular apertures 33 in the magnetic frame 31 permit thepassage of light from the source 23 through the optical system. Inpractice it is desirable to mount the light source 23 in as closeproximity to the ribbon 22 as is possible. Accordingly, while for thepurpose of simplifying the drawing the light source is shown outside themagnetic frame 3l, it will be understood that it may be mountedconveniently i side the frame. The ribbon 22 is so mounte that itspla-ne is parallel to the magnetic field produced by the valve winding.The rectified current in the output of the rectifier 21 may be sopolarized that the ribbon will be deflected either toward or away fromthe light source 23 an amount which is .proportional to the current,thus varying the total-quantity of light falling on the film 28. In thefollowing description of the operation of the electro-optical system ofFig. 1 it will be assumed that the rectified current in the output ofthe rectifier 21 is so polarized that an increase in this current willcause the ribbon22 to be deflected toward the light source 23. Undersuch conditions the picture will be reproduced on the film 28 in thesame form as it exists on the sending film 11. For example, if thesending fihn 11 is a positive the reproduced film 28 will also be apositive, and vice versa. In order to obtain this desired result it isunderstood that the apparatus at the transmittin terminal is so arrangedthat the dark condition of the photo-electric cell 7 results in themaximum amplitude of the,

modulated carrier -wave which is to ,be transmitted, while the maximumlight condition of the photoelectric cell results in the minimumamplitude of the modulated carrier wave.

In t-he diagram of Fig. 3, the diameter of the circular opening 24 inthe ribbon 22 is desi nate-d z., and the axis a of the lirlit-lx-aul mars the radius 1 of the area of illumination on the lens 25. Let a:represent the distance from the source of light 23 to the ribbon 22 atany instant, and let y represent the distance from the source of lightto the lens 25. The light which passes through thc opening 24 in theribbon is inversely proportional to the square of the distance Since thelens 25 focuses the light at all times at such a point as to distributethe light uniformly over the aperture in the screen 26, the change inthe intensity of the illumination of this aperture and its image uponthe light sensitive film 28 will bea function of the distance w of theribbon from the source of light 23. Assume that :ar- 0 when the currentI is at a maximum, a decrease in I causing to increase proportionately.Then a: varies inversely as the current I which may be expressed as Itis assumed that the beam of light from the source 23 has a constantintensity per unit area at a given cross-section of the beam, and hencethe total quantity of light Azv collected by the lens 25 is a functionof the cross-sec.- tion, or,

Az=Pr2 (4) and substituting in Equation (4) for the -value of 1-obtained in Equation (3) P 2711212 Azz-27,2 which -may also be Written l2 2 ifa-1% 1.2 (e

It was shown above that the current L, in the output of the translatingcircuit is equal to ,E a' being the current received from thetransmission line ML. Substituting for the current L, in yEquation (6),

l I P 2h2* r2 Xt,

from .which it will be seen that the total quantity of lightcollected bythe lens 25 is proportional to the current z' received from thetransmission line. ML. Since the density of light within the aperture inscreen 26 varies in direct proportion to the amount of light transmittedfrom the lens 25, the total quantity of light falling on the lightsensirent z' received from the transmission line ML.

It Was pointed out above that in the system of Fig. 1 the rectifiedcurrent in the output of the rectifier 21 is so polarized that anincrease in this .current Will cause the ribbon 22 to be deflectedtoward the light source 23, and that therefore the picture reproduced onthe 4'iilin 28 will be in the same form as the picture on the sendingfilm 11. If desired,

' source 23, in which case the picture reproduced on the film 28 will bethe reverseof the f picture on the sending iilm 11. This arrangement maybe employed when it is desired to reproduce a posit1ve from a negative,or vice versa. Whichever arrangement is used, however, the translatingsystem shown in Fig. 4

may be employed to produce the reverse effect. In the system of Fig. 4it will be assumed that the rectified current in the output of therectiier 21 is so polarized that an increase in this current will causethe ribbon of the lightvalve to be deflected away from the light source.In the system shown in F ig. 1 this arrangement would result inreproducing a picture on the lm 28 which would be the reverse of thepicture from which transmission was effected at the transmittingterminal, that is, if the sending film 11 .Were a positive the receivinglilm 28 would be a negative. By

means of the translating system shown in Fig. 4, however, an inverseeffect is produced whereby the picture reproduced on the tilm 28 is inthe same form as the picture on the sending film 11. f

In the system shown in Fig. 4, the output of the translating circuit 19is coupled by means of a transformer 34 to a balanced modulator 35 whichmay be of the type disclosed in Patent No. 1,449,382 to J. R; Carson,issued March 27, 1923. This modulator comprises a pair of similar spacedischarge tubes ,36 and 37 having divided input and output circuitslconnected ,in push-pull relation. The.

receiving circuit also includes an oscillator 38 for producing carrierwaves of the same frequency as the Waves produced by oscillator 6 at thetransmitting terminal, namely 1300 cycles frequency. The oscillator 38has its output circuit coupled by means of transformer 39 to the inputcircuit of a space discharge tube amplifier 40. The output circuit ofthe amplifier 40 is connected by means of transformer 41 andpotentiometer 42 to therectilier 21 which supplies rectified current tothe ribbon of the light valve. A branch circuit connected across thesecondary Winding of transformer 41 supplies carrier waves of 1300cycles frequency through transformer 43 to the 'common branch oftheinput circuits of modulator 35. tive film 28 is likewise proportional tothe cur- The balanced output circuits of the modulator 35 are coupled bymeans of a transformcr 44 to a gain control circuit including a highpass filter HPF which is connected by means of a` resistance 45 to aspace discharge tube rectifier 46. The output circuit of the rectifier46 is connected across a resistance 47 in the input circuit of theamplifier 40.

A source of negative grid biasing potential 48 in the input circuit ofthe amplifier 40 is of such value that the amplifier tube 40 operates onthe curved portion of the output current'- grid voltage characteristic,whereby very small changes in the voltage across the resist- -ances 47produce large changes in the gain of the amplifier. A synchronizingcircuit 49 is connected between the primary winding of transformer 34and the oscillator 38 to maintain the output of the oscillator 38 at thesame frequency and phase as the modulated Waves fiowing through theprimary winding of the transformer 34. The translating circuit 49 may beof the type disclosed in Patent N o. 1,450,966 to H. A. Afl'el, issuedApril 10,

The principle of operation of the balanced modulator 35 is Well known,and need -not be repeated except so far as it is considered necessary inpointing out the distinguishing lfeatures of the present invention.Following generally the analysis given in the above mentioned Carsonpatent, if a voltage e1 is impressed upon the primary winding oftransformer 34 and a voltage e2 is impressed upon the primary Winding oftransformer 43, then the voltage eg impressed upon the grid of themodulator tube 36 may be expressed by the equation v el e1 cos wt -le2cos (wt -l- 0) (8) and the voltage eG impressed upon the grid of themodulator tube 37 may be expressed by the equation eG el cos wte2 cos(wt -I- 0) (9) The current l in the upper primary Winding of the outputtransformer 44 is represented by and the current z', in the lowerprimary winding of this transformer is represented by i2=a+beg+ceg2 (1i)in which equations, a, b and c are constants depending upon the tubecharacteristics.

The voltage e3 induced in the secondary winding of the transformer 44 iseffect-2), p

= K1() (en eG) K10 (@112 @02) (12) Let eg: a+ -Then eG a- Substitutimrthese values in"y Equation (12) The voltage 0 induced in the secondarywinding of the transformer 44 thus includes a component of the signalingfrequency, that is, a 1300 cycle component, and a. component of twicethe signaling frequency. B-y propeily choosing the constants of the highpass filter HPF the first component may be suppressed and there isavailable across the input circuit of the rectifier 46 a voltage whichis proportional to the instantaneous values of the voltages acrosstransformers 34 and 43. That is, if the high pass filter HPF is given acharacteristic usuch that the cut-off frequency is thel voltage eimpressed across thc resistance 45 is given by p a 2 Klceleg cos (Zot-I- 0) (14) Since w and 0 are constant, the rectified current 'i3 in theoutput circuit of'tlic rectifier 46 may be expressed by the equation Acondenser 50 in the output circuit of l the rectifier 46 is of such avalue -as to by-pass the high frequency components in the output gli therectifier. It is evident that the potential impressed upon the grid ofthe amplifier tube 40 is a function of the rectified current 3. LettingF(3) represent vthe change in the voltage impressed upon the grid of theamplifier tube 40 for a change 6(123) in the rectified current, therelation between these instantaneous changes may be expressed aS L) lf3be) (16) It is desirable that large changes be pi'oduced in the currentI 0 in the output circuit of the amplifier 40 for relatively smallchanges in the rectified current 3. I o may be expressed as tive voltageimpressed upon the grid of the ampliier tube 40, and hence Since A ismuch greater than I 0 it is evident that for practical purposes that is,the product of I o andi',L s approximately'equal to a constant, andhence the output currentJ [o is the reciprocal of the input current L..It follows that the output current [o is always equal to some constantdivided by the input acurrent In, orin other words,

v K2' I I.- In 23) and since the current I., is the same as the currentin the output of the translating circuit 19,

Pfr Az (2 6) which may also be written P2h2 `1 A A.,f-

It was shown in Equation (24) that K I 3. o Hence K 2 I02 fist! andsubstituting this value of I 2 in Equation (27) A Pfr i 47532 X32 tionalto thecurrenti received from the transmission line ML.

Since, as pointed out above, the density of light within the aperture inthe screen 26 Avaries in direct proportion to the amount of currentreceived from thetransmission line It will be obvious that the generalprinciples herein disclosed may be embodied in many other organizationsWidely different from those illustrated without departing from thespirit of the invention as defined in the following claims.

What is claimed is:

l. In an electro-optical system, the method of obtaining signalmodulated light from a steady light source which comprises passing lightfrom said source through an aperture,

and alternately increasing and decreasing the distance between saidsource and said aperture in accordance with signaling current.

2. In an electro-optical system, the combination with a source of light,a source of signaling energy, and a screen having an aperture thereinthrough which light from said light source passes, of means for settingup a variable field of force in accordance with energy from saidsignaling energy source for varying the distance between said lightsource and said aperture to produce signal modulated light.

3. n an electro-optical system, means for producing an image in responseto variable received current which comprises a light source, a screenhaving an aperture therein through which light from said source pass,and means for varying the distance between said source and said screenin response to said variable current.

4. An electro-optical system comprising a voo source of light, anelectrically conducting eleof useful light from said source, means forpassing a varying current through said element, and means or set-ting upa steady magnetic field to react with the field set up by said currentsto cause said element to vibrate to and from said light source.

6. An electro-optical system comprising a source of light ofsubstantially constant intensity, a light sensitive element, anelectrically conducting element between said source and said lightsensitive element through which varying signal current passes, and meansfor `ducting element for controlling the amount Y setting up a magneticfield-which reacts with the field of said vibratable element to causeythe latter to vibrate to and from said source.

7 An electro-optical system comprising a source of light ofsubstantially constant intensity, a light sensitive element upon whichlight from said source falls, a vibratable elecf trically conductingelement having an aperture therein through which light from said sourcepasses, means for impressing receiving varying energy upon saidvibratable element,

and means for setting 'up a field of force which reacts with a field offorce set up by.

means actuated by signaling current from said source for producinglight, the, instantaneous effective value of which is proportional tothe reciprocal of the square of the actuating current. y

10. In a signaling system, means for producing signaling current, andmeans for producing current therefrom which is proportional to thereciprocal of a. root of said first current.

11. A signaling system comprising mean for producing signaling current,means for producing therefrom a second current, the instantaneous valueof which is a root of said first current, and means for producingthere'- from a third current which is propm'tional to the reciprocal ofsaid second current.

12. In a signaling system, means for producing a. modulated carriercurrent, means for producing therefrom a second modulated carriercurrent of the same carrier frequency, said second current beingproportional to the reciprocal of the square root of said first current,and a receiving device controlled by said second current.`

13. In a signaling system, means for pro-i ducing a signal modulatedcarrier wave, and means for producing therefrom a second modulatedcarrier wave which is always proportional to the reciprocal of saidfirst-car rier wave.

14. An electro-optical system comprising means at the transmitter forscanning an object or image, means for producing a cariier current theamplitude of which varies directly as the tone value of the elementalareas of the scanned object or image, means for producing from saidcurrent a second current the instantaneous value of which is equal tothe square root 'of said first current, means for producing from saidsecond current a third modulated carrier current of the same carrierfrequency 'as said second current and which is proportional to thereciprocal thereof, and an optical reproducing system controlled by saidthird current.` l

15. In an electro-optical system, the method of obtaining signalmodulated light from a steady light source which comprises passing lightfrom said source through a clear aperture, and alternately increasingand decreasing the distance between said source and said apertureinaccordance with signaling current. x

16. In an electro-optical system, the combination with a source of lightand al screen having a clear a erture therein through which light fromsaid source passes, of means for setting up electrocally a variablefield of force for varying the distance between vsaid light source andsaid aperture.

In testimony whereof I have signed my naine to this specification this3rd day of February 1927.

LEONARD G. ABRAHAM.

